Orientation

Astronomy would be easier if the Earth stood
still. In fact, the Earth has several different motions which
astronomers long ago struggled hard to understand. One of these
motions is the Earth's rotation on its axis; another is
the Earth's revolution, or orbital motion, around the
Sun.

ROTATION

If you watch the night sky for a few hours, you will see that the
stars appear to rotate about a fixed point in the sky, known as the
north celestial pole (which happens to be near the star
Polaris). This motion is due to the Earth's rotation. As the spin of
the Earth carries us eastward at almost one thousand miles per hour,
we see stars rising in the east, passing overhead, and setting in the
west. The Sun, Moon, and planets appear to move across the sky much
like the stars.

Because of the Earth's rotation, everything in the sky seems
to move together, turning once around us every 24 hours. Ancient
astronomers explained this phenomenon by supposing that the Sun, Moon,
planets, and stars were attached to a huge celestial
sphere, centered on the Earth, which rotated on a fixed axis
once per day. Of course, this sphere does not really exist; the Sun,
Moon, planets, and stars all fall freely through space, and
only appear to move together because of the Earth's rotation.
Nonetheless, we still use the concept of the celestial sphere in
talking about the positions of stars.

The celestial pole is 21.3° above the horizon as seen from
Oahu. The point on the horizon directly below the celestial pole is
north, while the opposite direction is
south. If you face north, east is on your
right and west is on your left. Finally, the
Zenith is the point exactly overhead.

Since the apparent rotation of the celestial sphere is due to the
actual rotation of the Earth, the north celestial pole is exactly
overhead as seen from Earth's north pole. Likewise, every point on
the celestial equator will be, once every day, exactly
overhead from some point on the Earth's equator.

REVOLUTION

Over the course of a year, the Earth makes one complete orbit about
the Sun. As a result, the Sun seems to move with respect to
the stars, appearing in front of one constellation after another, as
shown in the diagram on p. 12 of Stars & Planets.
After one year, the Sun is back where it started. The Sun's annual
path across the sky is called the ecliptic.
Traditionally, the ecliptic was divided into twelve equal parts, each
associated with a different constellation of the zodiac. The planets
also appear to move along the ecliptic, although, as we will see, they
don't always move in the same direction as the Sun.

The night sky is just that part of the sky which we see when the
islands of Hawaii have turned away from the Sun. As we orbit the Sun,
different constellations are visible at different times of the year.
In September, for example, the evening sky is still dominated by
summer constellations like Cygnus and Sagittarius; by December, these
constellations will be low in the western sky, and winter
constellations like Taurus and Orion will be rising in the east. You
can get a `sneak preview' of the winter sky by staying up late, thanks
to the Earth's rotation. For example, the constellations visible at
8 pm in early December can also be seen at 2 am in early
September.

The Earth's axis of rotation is not parallel to its axis of
revolution; the angle between them is 23.5°. As a result, the
ecliptic is inclined by the same angle of 23.5° with respect to
the celestial equator. This misalignment causes
seasons; when the Sun appears north of the celestial
equator the Earth's northern hemisphere receives more sunlight, while
when the Sun appears south of the celestial equator the northern
hemisphere receives less sunlight.

If we could view the Solar System from a point far above the north
pole, we'd see the Earth revolving counter-clockwise about the Sun and
rotating counter-clockwise on its axis. The other planets would
likewise revolve counter-clockwise around the Sun, and most of them would
also rotate counter-clockwise. In addition, the Moon would appear to
orbit the Earth in a counter-clockwise direction, as would most other
planetary satellites.

TIME

In this class, we will use a 24-hour clock instead of writing `am'
or `pm'. Since our class meets in the evening, most of the times we
will record are after noon, and the 24-hour time is the time on your
watch plus 12 hours. For example, our class starts at 19:00 (= 7:00
pm + 12:00), and ends at 22:00 (= 10:00 pm + 12:00). Sometimes we
need to record the date and the time together; for example, our first
class begins at 08/25/05, 19:00.

Astronomers everywhere in world use a single time system to
coordinate their observations. This system is called Universal
Time, abbreviated as UT or UTC.
(Greenwich Mean Time, abbreviated GMT, is the same
thing as UT.) Universal Time is exactly 10 hours ahead
of Hawaii time. To convert 24-hour Hawaii time to UT,
you add 10 hours; if the result is more than 24, subtract 24 and go to
the next day. For example, our first observing session (weather
permitting) will be at 09/01/05, 19:00, or 09/02/05, 05:00
UT. To convert from UT to Hawaii time,
subtract 10 hours; if the result is less than 0, add 24 and go to the
previous day. For example, in the website

http://www.planetkc.com/bobgraze/40city.htm

you can confirm that observers in Honolulu will see the star
SAO 146629 occulted, or hidden by the Moon, at
11/11/05, 5:53:53 UT; that is 11/10/05 19:53:53
Hawaiian time.

As a rule, we will use Hawaii time in this class, and write times
and dates without any time zone. The `UT' symbol will
be used to indicate universal time.

ALL-SKY CHARTS

Astronomers represent the appearance of the entire sky as seen at
some particular place and time by drawing circular all-sky
charts. Unfortunately, it's hard to show the appearance of
the sky on a flat piece of paper, so reading an all-sky chart and
relating it to what you see in the sky is a little tricky. For
example, these charts distort the patterns of stars near the horizon,
so you may find it hard to recognize constellations from an all-sky
chart. The only way to correct this distortion is to break the sky up
into several separate charts (this approach, used in The Sky
Tonight, helps to find the constellations). For some purposes,
however, it's very convenient to show the entire sky in one
chart, so you should learn to read these charts. All-sky charts for
each month of the year appear in Stars & Planets, starting
on p. 24.

To read an all-sky chart, hold it in front of you with the side
labeled `N' at the top. Now imagine you are lying flat on your
back with your head pointing north; then east will be on your left,
south at your feet, west on your right, and the Zenith right in front
of you. Mentally stretch the disk of the chart so that it forms a
dome over your position. The positions of stars on this imaginary
dome now correspond to their positions in the sky.

If you are used to reading maps of the Earth, the east and west
compass points may seem to be reversed. On a
terrestrial map with north at the top, you would expect to find west
to the left and east to the right. However, a celestial map with
north at the top has west at the right and east at the left. The
reason for this reversal is that a terrestrial map shows a view
looking down at the Earth, while a celestial map shows a view
looking up at the sky. Astronomical charts usually have north
at the top and west to the right. When using a telescope, you'll
notice that stars drift toward the west as a result of the Earth's
rotation; this is a convenient way to determine the correct way to
view a star chart.

CELESTIAL COORDINATES

Note: this is an advanced topic. We won't need to use celestial
coordinates in this class, but you'll see them mentioned from time to
time.

Just as latitude and longitude can be used to specify any point on
the Earth's surface, two celestial coordinates can be
used to specify any point on the celestial sphere. Imagine starting
from the point on the sky the point where the Sun, moving north,
crosses the celestial equator (this is the point labeled `0 h' in
the chart above). To reach any given point on the celestial sphere,
you could first travel along the celestial equator, and then towards
one of the celestial poles, until you reach your destination. The
angle you've traveled along the equator is called the right
ascension; it's measured in units of hours, where 1 hour
= 15°. The angle you've traveled towards one of the poles is
called the declination; it's measured in degrees, with
positive declinations towards the north celestial pole, and negative
declinations towards the south celestial pole.

As already noted, celestial coordinates won't be needed in this
class. They're included here because they are used in Stars &
Planets. The book often gives celestial coordinates when
discussing stars; for example, if you look at the description of alpha
Orionis on p. 194, you'll see `5h 55m +7°.4' just
after the star's name. This means that alpha Orionis has a right
ascension of 5h 55m (just slightly less than 6 hours) and a
declination of +7°.4 (a little north of the celestial equator).
Celestial coordinates also appear on the constellation charts; for
example, see the chart of Orion on p. 195, which shows that Orion
lies across the celestial equator at about 5h 30m right
ascension.

WEB RESOURCES

There is a website where you can produce an interactive planetarium,
set up to show the sky now above Honolulu. Or you can choose other
dates and times, select other viewing sites, and zoom in on selected
areas. See
http://www.fourmilab.to/yoursky.
Created by John Walker.

REVIEW QUESTIONS

If you face north, which way does the celestial sphere appear
to rotate - clockwise or counter-clockwise?

If you see the full Moon rising in the east in the evening,
where would you expect to see it very early next morning?

What is 09/20/05, 7:35 UT in local 24-hour
time? What day of the week? Is it morning or evening?